The invention relates generally to the production of steel and, more particularly, to a process and arrangement for the production of steel using direct reduction.
The melting of scrap is known, for instance, from the German Pat. No. 1,800,610. Here, scrap is introduced into the top of a vertical melting chamber and melted near the bottom of the chamber by means of a burner extending into the chamber. The flame generated by the burner is centered with respect to the chamber and is of plate-shaped configuration. There is no provision, however, for the production of steel where direct reduction of the starting materials is required.
With the known reduction processes which may be generally classified as "direct reduction processes", it is possible to transform ore or pellets to iron or iron sponge. These direct reduction processes are carried out in the most diverse types of apparatus. The best known are direct reduction processes carried out in shaft furnaces using lump ore or pellets and direct reduction processes carried out in rotating tubular furnaces using fine ore and coal. The end product of the direct reduction is iron sponge. Generally, the iron sponge still contains small residual amounts of unreduced oxygen and, in addition, retains as an impurity component the gangue-like components of the ore which cannot be reduced. This intermediate product, namely, the impure sponge iron, is, therefore, mostly used in a second, independent processing step wherein it is utilized as a starting material, so to speak, for scrap in the conventional steel production apparatus.
There is, however, a disadvantage associated with this use of the impure sponge iron. This resides in the fact that the sponge iron exhibits a tendency to re-oxidize by virtue of its great porosity, i.e. by virtue of the large exposed surface area of the sponge iron. Even at low temperatures, this tendency for re-oxidation may lead to ignition of the iron sponge. As a consequence, extended charging times during which the sponge iron is unprotected from the surrounding atmosphere, as well as smelting methods which take place under oxidizing conditions, lead to unfavorable results. Due to the latter reason, iron sponge which is produced by direct reduction processes is melted predominantly in electric furnaces. Here, continuous charging methods for introducing the sponge iron into the furnace have been found to work better than the otherwise conventionally used discontinuous, bucket charging methods. However, even though the use of electric furnaces for melting the sponge iron overcomes, at least to some extent, the problem of re-oxidation of the sponge iron, there still exists the disadvantage that the production of steel must proceed in at least two distinct stages, namely, a first stage where the sponge iron is produced and a second stage where the sponge iron is melted in an electric furnace after having been conveyed thereto from the furnace used for its production.
Another process is known from the U.S. Pat. No. 3,615,351 where a shaft furnace is used. A column of sponge iron is formed in the furnace and the sponge iron is melted with air burners under oxidizing conditions with a concomitant formation of slag. The furnace is greatly widened at the bottom thereof and molten material collects at the bottom of the furnace. Although the production and melting of the sponge iron are here carried out in the same furnace so that the sponge iron need not be conveyed to a separate furnace, the facts that melting is performed under oxidizing conditions and that slag is formed, present certain problems. Moreover, the thermal efficiency of this process is low since the waste gases are removed in the melting region and the heat carried thereby is lost. The thermal efficiency is further reduced because the column of sponge iron sinks into the molten metal on the bottom of the furnace. As a result, large quantities of heat are removed from the molten metal so that it becomes difficult to maintain the melt at a sufficiently high temperature to permit removal thereof from the furnace by tapping. Also, the period for which the molten metal remains in the shaft furnace and is collected at the bottom of the furnace is relatively long so that substantial amounts of oxidation occur. Furthermore, since the process proceeds discontinuously, the heat-resistant material of the furnace is greatly stressed. In addition, the sponge iron produced here has closed pores so that it is not possible to pass gases through the sponge iron, even if this were desired.